NEW YORK — Nearly seven months after a deadly explosion at the World Trade Center and three months after an abortive attempt to blow up the United Nations, the trial of four suspects charged in last February's bombing opens with jury selection today amid heavy security in a Manhattan federal courtroom.

The trial is expected to focus primarily on the physical evidence--the fingerprints, chemical traces, phone records and bank accounts--that prosecutors contend links the defendants to what has been characterized as the worst terrorist attack in U.S. history.

But motives and the question of whether a plot was sponsored by foreign groups or governments are likely to remain shrouded in mystery for now, according to sources close to the case.

A more complete picture of the scope of the alleged conspiracy will have to await a second trial early next year of more than a dozen other defendants, including a radical Muslim sheik. They will be tried on charges that they participated in a more ambitious terrorist scheme involving planned attacks on U.N. headquarters and New York commuter tunnels and assassination plots against foreign and domestic leaders.

This first trial will involve three suspects with Palestinian roots--Mohammed A. Salameh, Nidal Ayyad and Mohammad Ahmad Ajaj--and a fourth defendant born in Egypt, Mahmud Abouhalima. Two others under indictment in the trade center bombing are fugitives for whom the government has offered more than $2 million in rewards, and another has been severed from the case to face a separate trial.

Salameh, the first suspect arrested several days after the bombing, is accused of renting the yellow van that was used to carry the 1,200-pound homemade bomb into an underground garage at the trade center. Six people were killed in the Feb. 26 blast, more than 1,000 were injured and tens of thousands fled the scene, many descending smoke-filled stairways of the twin, 110-story towers.

Federal authorities contend that Salameh tripped himself up when he returned to the Ryder truck rental agency in Jersey City, N.J., soon after the blast to request a cash refund on grounds that the van had been stolen from him by unknown persons.

But Robert E. Precht, Salameh's court-appointed attorney, said that his client and other defendants will have "a good explanation" at trial for government evidence that seems to implicate them. Precht declined to give details. All four have pleaded innocent.

Defense lawyers have complained that massive publicity about the bombing case and the resulting anxieties of many New Yorkers will make it difficult for their clients to obtain a fair trial.

"One of my concerns is public hostility toward Arab persons, Palestinians, Muslims and aliens in general--and these defendants fall into that group," said Austin Campriello, another member of the defense team.

U.S. District Judge Kevin T. Duffy, who will preside at the trial, said that he considered moving the proceedings to Cincinnati in hopes of finding jurors less familiar with the case but decided that the effort would be pointless since the bombing had received international publicity.

Instead, Duffy has summoned a potential jury pool of 5,000 people, believed to be the largest in New York history. Selection of 12 suitable jurors could take three to four weeks. The trial itself may stretch out through the remainder of the year.

The defendants are followers of Sheik Omar Abdul Rahman, a blind Egyptian cleric whose fiery sermons in the New York-New Jersey area long have inspired supporters of Islamic fundamentalism. Abdul Rahman was indicted last month as a leader of the larger conspiracy.

Harry C. Batchelder, the sheik's defense attorney, said that he believes federal prosecutors hope to convict Salameh and the other bombing suspects to try to obtain their testimony later against the sheik and others accused in the larger conspiracy.